1. Field of the Invention
The invention relates to a method for surface modification of objects made of polyvinylidene fluoride and to the use of objects that have thus been modified, together with metals and other polymers, especially with glass-fiber reinforced materials, for the production of composite structures and laminates of high bond strength and shear strength.
2. Prior Art
Polyvinylidene fluoride has a high chemical resistance to inorganic acids, salt solutions and even to strongly oxidizing substances such as chlorine or bromine solutions. Its resistance to chemicals is comparable to that of polytetrafluoroethylene and other perfluorinated polymers. Furthermore, polyvinylidene fluoride can favorably be processed from the melt, extruded into tubes, panels, plates, sheets and other semifinished products and injection molded to give molded parts. Because of these properties polyvinylidene fluoride is a suitable material for the production of chemical containers, vessels, tanks, reactors and pipings, i.e., for any place where chemically aggressive fluids, acids or salt solutions are to be transported and stored. Tubes, pipings, receptacles and vessels made from pure polyvinylidene fluoride, however, often do not have the sufficient stability necessary for the manufacture of chemical appartus and equipment. Therefore, composite materials, for example, of polyvinylidene fluoride and metals or of polyvinylidene fluoride and glass-fiber reinforced polymers such as polyesters or epoxy resins have to be used. Also lacquer coatings with a high content of polyvinylidene fluoride disperson or laminates of polyvinylidene fluoride with polymers are used for the chemical resistant lining of steel tanks, vessels and pipings.
The stability and strength of these composite structures and laminates is, however, affected by the anti-adhesive behavior and the almost non-existent adhesion of polyvinylidene fluoride and also by the high coefficient of thermal expansion of the polymer which, in composite structures with metals or glass-fiber reinforced polyester or epoxy resins, causes a separation due to the high shearing forces occuring at high or low temperatures. In order to eliminate these disadvantages, copolymers of polyvinylidene fluoride, for example, with polymethyl methacrylate, have often been used as synthetic resin dispersions for impregnations and coatings. These copolymers, however, do not exhibit the high resistance to chemicals of the pure polyvinylidene fluoride and often have a poor abrasion resistance.
According to other known methods, glass fibers or organic synthetic fibers, such as fibers or fleeces of polyesters or polyamides, are embedded in polyvinylidene fluoride, in order to form bonding centers for the production of laminates and other composite materials. Also specially etched metal oxides or fibers can be used for the production of composite materials by pressure them into the surface of the object made of polyvinylidene fluoride.
Also adhesion-promoting resins, such as dissolved polyurethane or polyalkyl methacrylate, are known which, upon application under heat and in combination with a solvent which swells polyvinylidene fluoride render possible the thermowelding or bonding of polyvinylidene fluoride with metals or with other polymers. The realization of these efforts, i.e., the application of adhesion promoters or adhesives onto the pure polyvinylidene fluoride, results in composite materials which nearly do not resist thermal or mechanical loadings. Furthermore, the pressing of polyester fibers or glass fibers into polyvinylidene fluoride does not yield in products having high composite stabilities and, moreover, represents a complicated method.
Further complicated methods are also known which can be used only on a small scale, for example, grafting of polymethyl methacrylate after radiation with .gamma. quants, surface treatments by means of plasma techniques or oxyfluorination with O.sub.2 /F.sub.2 fumigation.